Waste Not, Want Not: Researchers Turn Waste Heat Into Energy

Just as frugal cooks use every possible scrap of food in their kitchens to avoid waste, so industrialists try to get the most out of their own processes. One way to do that is to tap the huge quantities of waste heat created in factories and power plants as a byproduct of mechanized operations.

Harnessing this form of heat hasn’t been easy. Until now, the focus has been on solid-state thermoelectric mechanisms, which can generate electricity from temperature differences. The materials for these devices, though, tend to be hard to find.

But researchers at the Massachusetts Institute of Technology and Stanford University report that they’ve found a new way to convert this waste heat to energy when these heat differences are less than the temperature of boiling water, 100 Celsius or 212 Fahrenheit.

Waste heat of 100 C or greater is efficient to use because it can boil water and thus create steam energy. But a large amount of waste heat falls below this temperature, making its use in energy generation a challenge.

And this low-grade heat is plentiful. Researcher Yi Cui of Stanford says, “Virtually all power plants and manufacturing processes, like steelmaking and refining, release tremendous amounts of low-grade heat to ambient temperatures. Our new battery technology is designed to take advantage of this temperature gradient at the industrial scale.”

According to MIT post-doctoral researcher Yuan Yang, “One-third of all energy consumption in the United States ends up as low-grade heat.”

The researchers say their technique can be applied to store energy in batteries, particularly rechargeable batteries, which have become a big focus energy research in the past few years. The MIT-Stanford researchers’ method is based on a phenomenon known as the thermogalvanic effect.

The voltage of rechargeable batteries relies on temperatures, and their solution combines the cycles of charging and discharging with heating and cooling, with the voltage during discharge greater than the voltage during charge.

This results in a net increase in energy because it delivers more electricity than was used to charge it. The report says this can be achieved at waste heat temperatures as low as 50 C (122 F). In a test using waste heat of 60 C (140 F), the new system had an estimated efficiency of 5.7 percent.

Gang Chen, an MIT professor, says he and his colleagues didn’t come up with the idea of using low-grade waste heat. It’s been around for six decades. Instead, he says, their contribution was “using material that was not around at that time” for the battery electrodes, as well as advances in engineering the system.

Yang adds that previous studies were based on temperatures of at least 500 C (932 F), because most current heat-recovery systems work best with higher temperature differences.

This new system’s advantage in converting waste heat to electricity has one downside: It has a relatively low power density, i.e., the quantity of power generated for a given weight. And Chen says additional research is needed to accelerate the charging and discharging of a batter and to assess its long-term reliability.